CT90 big bore...

Discussion in 'Battle Scooters' started by MNzach, Jun 4, 2014.

  1. MNzach

    MNzach Adventurer

    Joined:
    Nov 4, 2013
    Oddometer:
    13
    Location:
    Next to the Lake
    #1
  2. fullmetalscooter

    fullmetalscooter Let me take this duck off

    Joined:
    Apr 28, 2009
    Oddometer:
    3,306
    Location:
    BC

    From my reading here most rather go to dr.atv and do an engine change out by getting one of the kits they sell . That way you end up with a replacement engine to save or sell.
    #2
  3. SRG

    SRG SRG

    Joined:
    Nov 18, 2004
    Oddometer:
    1,338
    Location:
    Charlottesville, Virginia
    If you swap out the motor (for a non original one) you loose the dual range transmission - which is a big part of the CT's appeal.

    I did the DRATV big bore, performance cam & 22mm carb to my '75. I'm happy w/ it but for the price of that ebay big bore kit, I might have done that instead. You'll still want the Dr's cam and a 22mm carb.
    #3
  4. MNzach

    MNzach Adventurer

    Joined:
    Nov 4, 2013
    Oddometer:
    13
    Location:
    Next to the Lake
    Yea, the dual range is the main reason I haven't just swapped a Lifan into it. I think I'm gonna give this a shot whenever I get free time again. If I don't like how it works I can always put the stock cylinder back in.... but I doubt that would happen.
    #4
  5. oldxr

    oldxr Long timer

    Joined:
    Jan 28, 2007
    Oddometer:
    1,152
    Location:
    central komifornia
    The dratv piston is 53mm or 54 mm.That kit has a 52 mm cylinder.The head gaskit in that kit does not look correct for a ct90.If you make the engine bigger you will have to rejet the oem carb or get a new carb+ possably do some jetting work.I have a 1969 sl90 with the dratv 54mm piston+cam .The engine had a 22mm carb oem.My jetting is 92 main 38 pilot.With the 54mm piston you get 104cc.With a 52mm piston you will get 96+cc.The ct90 engine needs more stroke.I am working on a 125cc+ engine using a atc125m crank.The air intake in the rack is restrictive.The easiest hop-up I ever did on a ct based bike was a ct110.I removed the rubber boot from the rack and the baffle from the muffler.Then I put in a bigger main jet.The owner was very happy.
    #5
  6. MNzach

    MNzach Adventurer

    Joined:
    Nov 4, 2013
    Oddometer:
    13
    Location:
    Next to the Lake
    Thanks for the info! I didn't make the connection to the different size compared to the dratv piston. I'll have to do some more research before I do anything.
    #6
  7. injuhneer

    injuhneer Adventurer

    Joined:
    Nov 24, 2010
    Oddometer:
    21
    Location:
    SE Arizona
    So what did you finally do? I am working on a 146cc dual range motor for my ST-90. The thread is over in the "Old's Cool" forum.
    #7